Pulsed laser arrangement

ABSTRACT

A pulsed second harmonic output is obtained from a continuously pumped laser at repetition rates in the 1-10 MHz range. This is accomplished by electrically pulsing an intracavity nonlinear element to establish momentarily the phase-matched condition required for second harmonic generation. The generated harmonic is a high-amplitude optical pulse which is coupled out of the cavity via a harmonic-transparent mirror.

United States Patent Chesler et al.

[54] PULSED LASER ARRANGEMENT [72] Inventors: Ronald Benjamin Chesler,Summit; Joseph Edward Geusic, both of Berekely Heights, NJ.

[73] Assignee: Bell Telephone Laboratories Incorporated, Murray Hill,NJ.

[22] Filed: April 12, 1971 21 Appl. No.2 133,210

[151 3,684,893 51 Aug. 15, 1972 Primary Examiner-John Kominski AssistantExaminer-Darwin R. Hostetter Attorney-R. J. Guenther and Kenneth B.Hamlin [5 7] ABSTRACT A pulsed second harmonic output is obtained from acontinuously pumped laser at repetition rates in the 1-10 MHz range.This is accomplished by electrically pulsing an intracavity nonlinearelement to establish momentarily the phase-matched condition requiredfor second harmonic generation. The generated har- [52] U5. Cl...307/88.3, 321/69 R monic is a g -amp u optica pulse which is cou- 51int. Cl. ..n02m 5/06 p out of the cavity via a harmonic-transparent [58]Field of Search ..307/88.3; 321/69 R [56] References Cited 6 Claims 3Drawing figures UNITED STATES PATENTS 3,609,389 9/1971 Bjorkholm..307/ss.3

I 40 PULSE (I5 GENERATOR PUMP SOURCE l I J K 25 I I 45 i i i 1 LASER XUTILIZATION ELEMENT z Z APPARATUS L10 NONLINEAR 30 PULSED SECOND ELEMENT2O HARMONIC OUTPUT AMPLITUDE PATENTEDIUB I I972 F/G. A. 40

PULSE GENERATOR PUMP SOURCE I I I K I 45 T T T LASER X UTILIZATIONELEMENT Z Z APPARATUS f I 1, NONLINEAR 30 PULsEO sEcONO ELEMENT 20HARMONTc OUTPUT 40"" PULSE 2 GENERATOR NONLINEAR ELEMENT 2O (IO LASERPULsEO SECOND ELEMENT HARMONTE OUTPUT T f \L l l PUMP SOURCE -15 FIG. 3

APPLIED ELEO A A PULsE sEcONO HARMONIC A A A OUTPUT RB. CHESLERCIRCULATING INVENTORS J E. GEUS/C FUNDAMENTAL il e ga ATTORNEY PULSEDLASER ARRANGEMENT This invention relates to signal translation and moreparticularly to obtaining a pulsed second harmonic output from acontinuously pumped laser arrangement.

BACKGROUND OF THE INVENTION It is known to double the characteristicoutput frequency provided by a laser by combining therewith a materialthat exhibits a nonlinear optical effect. One advantageous suchnonlinear optical material is barium sodium niobate (Ba NaNb which, whencombined, for example, with a neodymium-doped yttrium aluminum garnet(NdzYAlG) laser is effective to constitute a particularly efiicientharmonic generator. In such a generator the characteristic 1.06-microninfrared output of the NdzYAlG configuration is converted to a visible(green) output at 0.53 microns, as described in The Nonlinear OpticalProperties of Ba NaNb c5 by J. E. Geusic, H. J. Levinstein, J. J. Rubin,S. Singh and L. G. Van Uitert, Applied Physics Letters, Nov. 1, 1967,pp. 269-271.

Moreover, it is known that pulsed operation of a frequency-doubled lasercan be achieved by including an additional element, specifically anacousto-optic Q- switch, in the optical cavity. An NdzYAlG laser adaptedfor Q-switched second harmonic generation is shown in FIG. 25 of AnExperimental and Theoretical Study of High-Repetition Rate Q-SwitchedNd:YAlG Lasers, by R. B. Chesler, M. A. Karr and J. E. Geusic,Proceedings of the IEEE, Dec. 1970, pp. 1899-1914. The depictedarrangement includes an NdzYAlG element, a Ba NaNb O nonlinear crystaland an acoustooptic Q-switch positioned along the main axis of thearrangement between cavity-defining mirrors.

Another NdzYAlG laser generator adapted for Q- switched second harmonicoperation is shown in FIG. 1 of Repetitively Q-Switched NdrYAlG LiIO0.53-Micron Harmonic Source by R. B. Chesler, M. A. Karr and J. E.Geusic, Journal of Applied Physics, Sept. 1970, pp. 4125-4127. In thatarrangement the N dzYAlG element and the acousto-optic Q-switch arepositioned along the main axis of the generator. But the nonlinearcrystal, lithium iodate (LiIO rather than Ba NaNb c5, is disposed in anolT-axis path. (The advantages of using such a folded cavity for secondharmonic generation are set forth in a copending application of R. B.Chesler, Ser. No. 818,962, filed Apr. 24, 1969, now US. Pat. No.3,628,045, issued Dec. 14, 1971.).

As a practical matter, acousto-optic Q-switched second harmonic lasergenerators of the NdzYAlG type are limited in operation to repetitionrates in the range l-lOO kHz. Axial mode-locking has been employed topulse NdzYAlG in a stable and efficient manner in the 300 MHz range. Butprior to the present invention, no practical and efficient way ofpulsing Nd2YAlG lasers at repetition rates in the range between theQ-switched and mode-locked ranges was available.

SUMMARY OF THE INVENTION An object of the present invention is animproved laser arrangement.

More specifically, an object of this invention is a practical andefficient continuously pumped laser arrangement adapted to provide apulsed second harmonic output at a repetition rate of approximately ll 0MHz.

These and other objects of the present invention are realized in aspecific illustrative embodiment thereof that comprises a continuouslypumped laser element positioned in an optical cavity that includes amirror element transmissive to second harmonic radiation. Also includedin the cavity is a nonlinear element which can be phase-matched forsecond harmonic generation. Quiescently, the temperature of thenonlinear element is selected such that the phase-matching conditionrequired for second harmonic generation is not satisfied. Under suchconditions a fundamental signal is sustained within the cavity but nosecond harmonic signal is available to be coupled out of the cavity.

Pulsing the nonlinear element with an electrical signal of a specifiedamplitude alters the phasematching factor within the element in apredetermined manner by means of the electro-optic effect. In this waypower at the fundamental is converted to the harmonic during thepersistence of the applied electrical signal. In turn, the harmonicpulse is coupled from the cavity by the harmonic-transmissive mirrorelement. As a result, a substantial portion of the stored energy of thecavity is dumped, the output of the arrangement being a narrow opticalpulse whose peak power is determined by the quiescent intracavitycirculating power.

A feature of the present invention is that the phasematching conditionrequired in a nonlinear optical element for second harmonic generationis selectively controlled to achieve a variable output coupling for thesecond harmonic signal from a laser cavity that contains the nonlinearelement.

Another feature of this invention is that the phase matching conditionrequired for second harmonic generation be established in the cavity byapplying an electrical pulse signal to the nonlinear optical element.

BRIEF DESCRIPTION OF THE DRAWING A complete understanding of the presentinvention and of the above and other objects, features and advantagesthereof may be gained from a consideration of the following detaileddescription of several specific illustrative embodiments thereofpresented hereinbelow in connection with the accompanying drawing, inwhich:

FIGS. 1 and 2 respectively depict pulsed laser arrangements made inaccordance with the principles of the present invention; and

FIG. 3 shows some waveforms that are helpful in understanding the modeof operation of the depicted arrangements.

DETAILED DESCRIPTION The specific arrangement shown in FIG. 1 includes alaser element 10 continuously pumped by a source 15. Illustratively, theelement 10 comprises an NdzYAlG rod and the source 15 includes atungsten lamp. In one practical arrangement the element 10 and the lampare placed at the respective foci of a gold-plated elliptical cylinder(not shown) which constitutes a pumping cavity.

In Nd:YAlG, oscillation can be achieved corresponding to a number ofinfrared transitions, the most prominent room-temperature transitionoccurring at 1.06 microns. Herein, for illustrative purposes,oscillations will be assumed to occur at 1.06 microns.

In addition, it is well known that visible outputs from an NdzYAlG lasercan be obtained by combining therewith a nonlinear material which can bephasematched for second harmonic generation. The FIG. 1 arrangementincludes such a nonlinear material, the element 20. Illustratively, theelement 20 is a crystal of Ba NaNb c5. By means of the element 20 it ispossible to convert a substantial portion of the available output at thefundamental (1.06 microns) to the harmonic (0.53 microns).

Both of the elements and 20 of FIG. 1 are contained in an optical cavitydefined by a concave mirror 25 and a planar mirror 30, the mirror 30being disposed, for example, on the right-hand end of the element 20.The mirror 25 is selected to be highly reflective to both fundamentaland harmonic signals directed thereat, whereas the mirror 30 is selectedto be highly reflective to the fundamental but highly transmissive tothe harmonic.

Advantageously, in a manner well known in the art, the optical cavityrepresented in FIG. 1 is designed so that the laser element 10 itselfprovides sufficient aperturing to allow oscillation in the cavity onlyin the fundamental (TEM transverse mode.

If the phase conditions that are known to be required for secondharmonic generation are not quiescently satisfied in the nonlinearelement 20, only the fundamental signal at 1.06 microns oscillateswithin the cavity. The fundamental propagates through the nonlinearelement 20 along the axis designated X, Z indicating the optic axis ofthe element 20. In such a phasemismatched state, the element 20 isessentially transparent to the fundamental. Since the mirror 30 ishighly reflective to the fundamental and since no harmonic existsquiescently in the arrangement, substantially no output is delivered toutilization apparatus 35 when the element 20 is phase-mismatched.

Illustratively, the aforementioned phase-mismatched condition isachieved quiescently by selectively controlling the temperature of thenonlinear element 20 of FIG. 1. (A.A.Ballman et al U.S. Pat. No.3,262,058, issued July 19, 1966, discloses that phase-matching in anonlinear medium depends on the medium evidencing a degree ofbirefringence adequate to compensate for dispersion in the medium and,furthermore, that the birefringence-dispersion relationship in themedium is strongly temperature-dependent.) The high birefringence anddispersion of Ba NaNb O permit phasematched second harmonic generationof 0.53 microns from 1.06 microns at temperatures slightly above roomtemperature. For light propagating in the X direction with thefundamental polarized parallel to the Y axis (which is perpendicular tothe plane of FIG. 1) and the harmonic polarized parallel to Z,phase-matched second harmonic generation utilizing (132 occurs in BaNaNb c5 at a temperature of approximately 80 C.

In accordance with the principles of the present invention, thetemperature of the nonlinear element 20 shown in FIG. 1 is purposelyadjusted to be a few degrees removed from the temperature at whichphasematching is realized. Thus, for example, if the element 20 is madeof Ba NaNb O a temperature of approximately 82 C. will achievequiescently a phasemismatched condition. As specified below, such amismatched condition can be compensated for on a controlled intermittentbasis in a unique and advantageous manner.

In the specific illustrative embodiment shown in FIG. 1 the coupling ofsecond harmonic signals out of the depicted cavity is modulated by thephase-matching factor siu AKZ 1) where l is the length (along the Xaxis) of the element 20, and AK= (4'n'f/c) (n n wherefis the fundamentalfrequency, 0 is the speed of light, and n, and n 8 are the indices ofrefraction in the element 20 for the fundamental and harmonic,respectively. The coupling can be varied from zero to its maximum valueby adjusting the temperature of the element 20 so that AKI 21r(phase-mismatched) and then applying an electric field which provides anadditional phase shift to make AKl 0. For light propagated along the Xaxis (utilizing (132 for harmonic conversion) and an applied field alongthe Z axis, a field-length product (El) of about 2,640 volts yields AKll 211-. Thus, for example, for a nonlinear element 20 having dimensions5 X 5 millimeters by l millimeter in the Z direction, the requiredvoltage to achieve the phase-matched condition is about (2,640)/(5) orapproximately 5530 volts. Accordingly, application of such a voltagepulse (see top row of FIG. 3) causes a substantial portion of thecirculating fundamental to be converted to the harmonic. In turn theharmonic is dumped out of the cavity via the harmonictransrnissivemirror 30 (see second row of FIG. 3)

In cavity-dumped operation as described above, the depicted laserarrangement stores energy in the optical cavity during the interpulseperiod when the output transmission of the mirror 30 is almost zero.During each pulse interval a portion of the circulating fundamental isconverted to the harmonic. During those intervals the consequentdecrease in the level of the circulating fundamental is represented inthe bottom row of FIG. 3. Between intervals the level of the circulatingfundamental reestablishes itself at a predetermined quiescent level.

In accordance with the principles of the present invention, anelectrical pulse generator 40 is connected to the nonlinear element 20(FIG. 1) to apply momentarily an electric field of a predetermined valuealong the Z axis. This field exactly compensates for the quiescentphase-mismatched condition and thus serves in effect to switch thedepicted arrangement momen tarily to its phase-matchedsecond-harmonic-generating state. Advantageously, a resistor 45 isconnected in parallel with the element 20 and the generator 40 tofacilitate fast decay of applied pulses. (For a dielectric constant K51, the capacitance of the element 20 is about 11 picofarads.) For arepetition rate M and a pulse width (D)/(M) (where D is the duty cycle),a time constant RC of approximately (D)/(5M) is adequate for repetitionrates in the l 10 MHz range. The average power supplied by the generator40 is given by (V D)/( R) SMV C. If, as a practical matter, anadditional 9 picofrarads is allowed for stray capacitance, the averagepower supplied by the generator 40 is about 28 watts per MHz ofrepetition rate.

Commercial generators are available to supply electrical pulses at thispower level in the l MHz range.

The range of repetition rates for which efficient harmonic cavitydumping is feasible can be expressed in terms of e, the decay timeconstant of the optical cavity represented in FIG. 1. The value of thisconstant is given by (2) where c is the speed of light, 2L is the roundtrip optical path length, and A is the round trip fractional loss. For atypical NdzYAlG laser, 2L approximates one meter, A approximates 0.01and 6 therefore is about 3 MHz. Efficient harmonic cavity dumping with aduty cycle D requires a coupling constant approximately (1)/(D times thecontinuous wave optimum value for repetition rates equal to or greaterthan 6. For repetition rates much less than 6, the required couplingrapidly becomes unrealizable. For example, at (e)/( 10), a couplingconstant roughly 3,300 times the continuous wave optimum value isrequired for efficient 10 percent duty cycle operation. The delayconstant e can be significantly decreased by using a lengthened orfolded delay line cavity (see, for example, Applied Optics, Vol. 3, No.4, p. 523, 1964). On the other hand, harmonic cavity dumping at or above6 is feasible with high-optical-quality Ba NaNb O Illustratively, forefficient operation at 1 MHz, a cavity optical length on the order oftwo meters suffices. The peak output power at 1 MHz varies linearly withthe nonlinear coupling strength employed. At 1 MHz, for a couplingstrength 10 times the continuous wave optimum value, the peak harmonicpower output is about times the continuous wave output, with a pulsehalfwidth of about nanoseconds.

An alternative embodiment of the principles of the present invention isshown in FIG. 2. The FIG. 2 arrangement includes a so-called foldedcavity of the type described in the aforecited Chesler application. Asis set forth in that application, such a folded cavity allows 100percent intracavity harmonic conversion and complete collection of thesecond harmonic in a single output beam. In such a design, the curvatureof mirror element 50 and other parameters of the depicted cavitygeometry can be chosen to independently adjust the beam radii in thelaser element 10 and in the nonlinear element 20.

The elements 10, 15, 20, 25, 40 and shown in FIG. 2 may be identical tothe correspondingly numbered units of FIG. 1. The FIG. 2 arrangementalso includes a cavity-defining planar mirror 55 which is designed to behighly reflective to both fundamental and harmonic signals. In additionthe aforementioned mirror element is selected to be highly reflective tothe fundamental but highly transmissive to the harmonic. In allessential respects, the overall mode of operation of the FIG. 2embodiment in providing a pulsed second harmonic output in response toelectrical signals provided by the generator 40 is the same as thatdescribed above in connection with the FIG. 1 arrangement.

It is to be understood that the above-described arrangements are onlyillustrative of the application of 6 the principles of the presentinvention. In accordance with these principles, numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example,although emphasis herein has been directed to a particular pulsed secondharmonic system including NdzYAlG and Ba NaNb O it is to be understoodthat a variety of other laser and nonlinear materials are available andsuitable for substitution therefor.

What is claimed is 1. Apparatus for providing pulsed second harmonicoutput signals comprising means defining a laser cavity,

pumping means,

means positioned in said cavity and responsive to radiation emitted bysaid pumping means for causing a fundamental signal to be propagated insaid cavity,

a quiescently phase-mismatched nonlinear element positioned in saidcavity in the path of said fundamental signal,

and means for momentarily establishing a phasematched condition in saidnonlinear element so that the fundamental signal directed at saidnonlinear element is converted to a pulsed second harmonic signal whichis dumped out of said cavity.

2. Apparatus as in claim 1 wherein said means for causing a fundamentalsignal to be propagated in said cavity comprises an element made ofNdzYAlG.

3. Apparatus as in claim 2 wherein said nonlinear element is made of BaNaNb O 4. Apparatus as in claim 3 wherein said means defining a lasercavity comprises first and second mirror elements spaced apart along amain longitudinal axis of said cavity and contain ing therebetween bothsaid means for causing a fundamental signal to be propagated and saidnonlinear element, said first mirror element being highly reflective tofundamental and second har monic signals directed thereat, said secondmirror element being highly reflective to fundamental signals but highlytransmissive to second harmonic signals directed thereat.

5. Apparatus as in claim 3 wherein said means defining a laser cavitycomprises first and second mirror element spaced apart along a mainlongitudinal axis of said] cavity and containing therebetween said meansfor causing a fundamental signal to be propagated, said first mirrorelement being highly reflective to both fundamen tal and second harmonicsignals directed thereat, said second mirror element being highlyreflective to fundamental signals but highly transmissive to secondharmonic signals directed thereat, said second mirror element beingstructured to direct incident main-axis fundamental signals along anoff-axis path and to direct incident off-axis fundamental signals alongsaid main axis,

and a third mirror element disposed in said off-axis path and spacedapart from said second mirror element, said nonlinear element beingpositioned in said off-axis path between said second and third mirrorelements, said third mirror element being highly reflective to bothfundamental and second harmonic signals.

6. In combination,

means defining a laser cavity, said means including a mirror elementtransmissive only to second harmonic signals,

- 3,684,893 7 8 laser element means positioned within said cavity pledout of said cavity,

and responsive to being pumped fo g ne g a and means connected to saidnonlinear element fufldamemal Signal Within l cavity, I means forapplying thereto an electrical signal to nonlinear element meanspositioned within said cavity and, when phase-matched, responsive tosaid fundamental signal for generating second harmonic signals, saidnonlinear element means being quiescently phase-mismatched so that saidfundamental signal passes therethrough with minimal nonlinearinteraction and minimal second harmonic generation whereby minimalenergy is couestablish therein the phase-matched condition required forsecond harmonic generation whereby during the application of saidelectrical signal a high-amplitude second harmonic optical pulse isgenerated and coupled out of said cavity via said mirror element.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,68 1,893 Dated August l5 1972 Inventofls) Rohsld B. Chesler and Joseph F.Geusj c It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

001. 1, line 19, delete "Ba NaNbSc5" and insert -Ba NaNb line l l,delete "Ba NaNb c5" and insert --Ba NaNb O V Q. n n i Col. 3, line 7,delete Ba NaNb 05 and nsert -Ba NaNb O line 58, delete "Ba NaNb c5" andinsert --Ba NaNb Col. 1,- line 29, after "approximately" delete "5530"and insert 530--. I

. Signed and sealed this th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer I Commissionerof Patents FORM PO'WSO uscoMM-oc 60376-F89 i U. 5 GOVERNMENT PRINTINGOFFICE: I959 0-366-334,

1. Apparatus for providing pulsed second harmonic output signalscomprising means defining a laser cavity, pumping means, meanspositioned in said cavity and responsive to radiation emitted by saidpumping means for causing a fundamental signal to be propagated in saidcavity, a quiescently phase-mismatched nonlinear element positioned insaid cavity in the path of said fundamental signal, and means formomentarily establishing a phase-matched condition in said nonlinearelement so that the fundamental signal directed at said nonlinearelement is converted to a pulsed second harmonic signal which is dumpedout of said cavity.
 2. Apparatus as in claim 1 wherein said means forcausing a fundamental signal to be propagated in said cavity comprisesan element made of Nd:YAlG.
 3. Apparatus as in claim 2 wherein saidnonlinear element is made of Ba2NaNb5O15.
 4. Apparatus as in claim 3wherein said means defining a laser cavity comprises first and secondmirror elements spaced apart along a main longitudinal axis of saidcavity and containing therebetween both saiD means for causing afundamental signal to be propagated and said nonlinear element, saidfirst mirror element being highly reflective to fundamental and secondharmonic signals directed thereat, said second mirror element beinghighly reflective to fundamental signals but highly transmissive tosecond harmonic signals directed thereat.
 5. Apparatus as in claim 3wherein said means defining a laser cavity comprises first and secondmirror element spaced apart along a main longitudinal axis of saidcavity and containing therebetween said means for causing a fundamentalsignal to be propagated, said first mirror element being highlyreflective to both fundamental and second harmonic signals directedthereat, said second mirror element being highly reflective tofundamental signals but highly transmissive to second harmonic signalsdirected thereat, said second mirror element being structured to directincident main-axis fundamental signals along an off-axis path and todirect incident off-axis fundamental signals along said main axis, and athird mirror element disposed in said off-axis path and spaced apartfrom said second mirror element, said nonlinear element being positionedin said off-axis path between said second and third mirror elements,said third mirror element being highly reflective to both fundamentaland second harmonic signals.
 6. In combination, means defining a lasercavity, said means including a mirror element transmissive only tosecond harmonic signals, laser element means positioned within saidcavity and responsive to being pumped for generating a fundamentalsignal within said cavity, nonlinear element means positioned withinsaid cavity and, when phase-matched, responsive to said fundamentalsignal for generating second harmonic signals, said nonlinear elementmeans being quiescently phase-mismatched so that said fundamental signalpasses therethrough with minimal nonlinear interaction and minimalsecond harmonic generation whereby minimal energy is coupled out of saidcavity, and means connected to said nonlinear element means for applyingthereto an electrical signal to establish therein the phase-matchedcondition required for second harmonic generation whereby during theapplication of said electrical signal a high-amplitude second harmonicoptical pulse is generated and coupled out of said cavity via saidmirror element.